can: mcp251x: add missing IRQF_ONESHOT to request_threaded_irq

Since commit:

    1c6c6952 genirq: Reject bogus threaded irq requests

threaded irqs must provide a primary handler or set the IRQF_ONESHOT flag.
Since the mcp251x driver doesn't make use of a primary handler set the
IRQF_ONESHOT flag.

Cc: linux-stable <stable@vger.kernel.org> # >= v3.5
Reported-by: Mylene Josserand <Mylene.Josserand@navocap.com>
Tested-by: Mylene Josserand <Mylene.Josserand@navocap.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>